1. Field of the Invention
The present invention relates to a ladder resistor circuit made of a multilayer of polycrystal silicon films, and a semiconductor device constituted by this ladder resistor circuit and a transistor.
2. Description of the Related Art
FIG. 2 is a sectional view for schematically showing resistor elements used in a conventional ladder resistor circuit, and a conventional insulated gate field-effect transistor (will be abbreviated as an "MISFET" hereinafter).
In this drawing, a polycrystal silicon film is formed via an oxide film 2 on a surface of a substrate 1. This polycrystal silicon film constitutes a gate electrode of the MISFET having a gate oxide film 3 formed by a portion of the oxide film 2, a source/drain region 4, and a gate electrode 5, and also a resistor element. High concentration impurity regions 6 are formed on both sides of the resistor element made of the polycrystal silicon film, and a low concentration impurity region 7 for determining a resistance value of the resistor element is formed between these high concentration impurity regions 6. A metal wiring line 9 is provided via a contact hole of an intermediate insulating film 8 on the high concentration impurity regions 6 of this resistor element, so that an ohmic contact with the high concentration impurity regions 6 can be maintained. Further, silicon nitride film passivation 10 is provided over the entire portion. A ladder resistor circuit is formed on the same substrate surface in such a manner that a plurality of resistor elements shown in FIG. 2 are connected via metal wiring lines in series to each other, or in parallel to each other.
However, conventionally, when the insulated gate field-effect transistor and the ladder resistor circuit are manufactured as the integrated circuit, there are the following problems.
That is, in the case that the resistance value of the resistor element constituted by the polycrystal silicon is designed to be large, the length of this resistor element must be made long in directly proportional to this large resistance value. Accordingly, the chip size is increased and thus the low cost circuit cannot be provided. In particular, in the D/A converter and the A/D converter where the area and the precision of the ladder resistor circuit may give great influences to the product cost and also the product performance, when the gate electrode of the insulated gate field-effect transistor and the ladder resistor are manufactured by the common thin film, which are formed on the same substrate, the area of the ladder resistor is increased. Therefore, the product cost is increased as well.